I caught a 18 and 3/4 inch rainbow in the canyon section below the falls on Abrams creek in 1986,this was before I started using a fly rod and was using stickbait on a size 10 eagle claw bait hook.In 94 I caught an 18 inch rainbow on a size 14 extended deer hair body parachute green drake that I tied using a recipe in Jack Dennis' Western Fly Tying book(still a great book all these years later).This fish was caught about 50 yds above the falls so he was not a lake run fish.
I caught a 17 inch rainbow on a 16 red humpy in Nov of 93 a couple of miles upstream from the lake on Hazel creek,I guess he could have come up from the lake,it was a very colorful fish that was heavily spotted,sort of like the ones I have seen in magazine articles about Alaskan ones called leopard rainbows.About a half hour later meet two sports with spinning rods who had a dead brown trout that was closer to 30 inches than 25 hangin on a stringer,truly a magnificent beast.
A guy that used to post here(creation bear)told me about catching a 17 in rainbow above camp 18 on the West Prong of Little river,I guess it might have come upstream after being stocked in Townsend.
The section of Cattaloochie from the park line to Waterville lake used to have some really big rainbows(and browns).I cannot imagine these fish swimming up from that nasty lake.
So,yeah I think there are rainbows that big in park waters that are wild streamborn fish.